This invention pertains to articles (e.g., an optical fiber communication system, or a light source or amplifier for such a system) that comprise an optical fiber Raman device.
Optical fiber Raman lasers and amplifiers (collectively xe2x80x9cfiber Raman devicesxe2x80x9d) are known. See, for instance, U.S. Pat. No. 5,323,404 for exemplary embodiments of fiber Raman devices of a first (topologically linear) type, with fiber gratings acting as wavelength-selective elements.
For a second type of fiber Raman laser (topologically circular), see for instance, S. V. Chernikov et al., Electronics Letters, Vol. 34(7), April 1998, pp. 680-681. This embodiment uses fused fiber couplers as wavelength-selective element to form a ring cavity. All cited references are incorporated herein by reference.
An optical fiber Raman device can be used to provide pump light (exemplarily of wavelength 1480 nm) to an Er-doped fiber amplifier (EDFA), or can be used to amplify signal light (e.g., at 1310 nm).
FIG. 1 schematically depicts a prior art fiber Raman laser 10 of the topologically linear type, suitable for pumping of an EDFA. Cladding pumped fiber laser (CPFL) 11 provides pump light of a predetermined wavelength (e.g., 1117 nm) to the Raman laser. Raman fiber 12 typically is a silica-based fiber with Ge-doped core, and typically is hundreds of meters long. Numerals 13 and 14 refer to the upstream and downstream grating sets, respectively. It will be appreciated that in a schematically depicted grating set herein each cross-line indicates a separate grating. The upstream set 13 typically comprises only high reflectivity (HR) gratings (exemplarily of center wavelengths 1175, 1240, 1315, 1395, and 1480 nm), and the downstream set 14 typically comprises, in addition to the HR gratings, also a grating of relatively low reflectivity, to provide output coupling. By way of example, the downstream gratings have center wavelengths 1117, 1175, 1240, 1315, 1395 and 1480 nm, with the 1117 nm grating serving as pump reflector. The output coupler has center wavelength corresponding to the desired output wavelength, exemplarily 1480 nm.
CPFLs are known, and are commercially available. See for instance, U.S. patent applications Ser. Nos. 08/897,195 and 08/999,429, respectively filed Jul. 21, 1997 and Dec. 29, 1997 by DiGiovanni et al. Briefly, a CPFL comprises several high power light emitting diodes (exemplarily InGaAlAs diodes). The output of each LED is coupled into a multimode fiber, e.g., a silica-based fiber with 0.22 N.A., 105 xcexcam core and 125 xcexcm outside diameter. The fibers are arranged into a bundle, fused together and tapered, as described, for instance, in US patent application Ser. No. 09/315,631, filed May 20, 1999 by D. J. DiGiovanni et al.
To date it has not been convenient to form tapered bundles of more than seven multimode fibers. This has limited the number of pump sources to seven, and has correspondingly limited the power that can conveniently be provided to utilization means, e.g., the EDFA.
It clearly would be desirable to be able to conveniently provide to the fiber Raman device pump light from more than seven LEDs. This application discloses an article that comprises a fiber Raman device that is pumped with light from more than seven pump LEDs, exemplarily 14 pump LEDs.
The terms xe2x80x9clightxe2x80x9d and xe2x80x9cradiationxe2x80x9d are used herein interchangeably for electromagnetic radiation of interest herein, typically infrared radiation.
Optical fiber gratings and fused fiber couplers are herein collectively referred to as xe2x80x9cwavelength-selective elementsxe2x80x9d. A fused fiber coupler is frequently referred to as a xe2x80x9cWDMxe2x80x9d.
The xe2x80x9cRaman spectrumxe2x80x9d of an optical fiber is the scattered intensity as a function of wavelength difference from an incident radiation. A shift to longer wavelength is generally referred to as a Stokes shift. Conventionally, the Stokes shift is expressed in inverse centimeters (cmxe2x88x921), but it can also be expressed in terms of wavelengths.
The Raman spectrum of germano-silicate glass is relatively broad, with a pronounced maximum at a Stokes shift of about 440 cmxe2x88x921, relative to the wavelength of the pump light. See FIG. 2 herein, which shows the Raman spectrum for pump light of 1427 nm.
A wavelength-selective element in a Raman device that is responsive to a given pump light is herein referred to as being xe2x80x9con resonancexe2x80x9d (with respect to the pump light), and an element that is not responsive to the given pump light is herein referred to as being xe2x80x9coff resonancexe2x80x9d (with respect to the pump light).
A wavelength-selective element is xe2x80x9cresponsivexe2x80x9d to a given pump light if the elementlight interaction is at or near maximum, for instance, if the pump light is within the wavelength range wherein the reflectivity of the grating is 50% or more of the maximum reflectivity of the grating, or wherein the coupling strength of a fiber coupler (WDM) is 50% or more of the maximum coupling strength of the coupler.
In a broad aspect, the invention is embodied in an optical fiber communication system or other article that comprises an optical fiber Raman device that is adapted for utilizing high pump power.
The Raman device comprises a length of silica-based optical fiber comprising at least a first and a second wavelength-selective element disposed to provide one or more optical cavities for Raman-shifting of light in said optical fiber, and further comprises a first coupler for coupling pump radiation of a first wavelength xcex1 from a first pump radiation source into said optical fiber, and still further comprises means for providing a Raman-shifted Raman device output radiation of wavelength xcex0 greater than xcex1 to output radiation utilization means. Significantly, the fiber Raman device further comprises a second coupler for coupling pump radiation of a second wavelength xcex2 from a second pump radiation source into said optical fiber, where xcex2 is different from xcex1, with xcex0  greater than xcex2, wherein at least one of said wavelength-selective elements is off resonance with regard to at least one of xcex1, and xcex2. 
If the fiber Raman device is a topologically linear Raman laser then the first and second wavelength selective elements typically are fiber gratings, and the means for providing the output radiation to utilization means exemplarily comprise an output coupler of relatively low reflectivity. If the device is a topologically circular Raman laser then the wavelength selective elements typically are fiber couplers (WDMs), and the means for providing the output radiation typically also comprise a WDM.
If the fiber Raman device is a topologically linear Raman amplifier then the wavelength selective elements typically are fiber gratings, and the means for providing the output radiation to utilization means comprise a high-reflectivity optical cavity for radiation that is one Stokes shift from a signal radiation. If the device is a topologically circular Raman amplifier then the wavelength selective elements typically are WDMs, and the means for providing the output radiation typically also comprise a WDM.
If the Raman device is a topologically linear Raman device then |xcex1xe2x88x92xcex2| typically is greater than 0.2 nm, and if the Raman device is a topologically circular Raman device then |xcex1xe2x88x92xcex2| typically is greater than about 3 nm.